1. Field of the Invention
The present invention relates to a print head, more particularly, it relates to a print head for ejecting ink with a thermomechanical actuator to perform printing.
2. Description of the Related Art
As an ink ejecting method of a print head used for ink jet printing apparatuses, the following methods are known and utilized: a method for generating bubbles by applying thermal energy to ink; a method for ejecting the ink with an electrical-mechanical actuator constituted by a piezoelectric device; and the like. Additionally, a method using a thermomechanical actuator has been developed in terms of simplicity in processing and high degree of freedom of an ink composition.
Japanese Patent Laid-Open No. 2003-260696 discloses a print head using a thermomechanical actuator constituted of two layers, a heat generation layer and a dielectric layer which constitute a cantilever. The thermomechanical actuator including the heat generation layer and dielectric layer constituting the cantilever will be briefly described with reference to FIGS. 8A to 8C.
FIG. 8A is a top plan view of an ejecting portion of the print head (the top view indicates that the ejecting portion is viewed from the side in which an ink droplet is ejected). FIG. 8B is a cross sectional view taken along line VIIIB-VIIIB of the ejecting portion of the print head shown in FIG. 8A. FIG. 8C is a view for illustrating a state where the ink droplet is ejected from the ejecting portion of the print head shown in FIGS. 8A and 8B.
As shown in FIGS. 8A and 8B, a liquid chamber 2 is formed on a silicon substrate 1, and the ink droplets are ejected from a nozzle 3. A cantilever 4 as the thermomechanical actuator is formed in the liquid chamber 2. The cantilever 4 includes: a heat generation layer 20 which is divided into two heat generating portions by a slit; a conductor layer forming wiring portions 5 (5a, 5b) for supplying power to the two heat generating portions, and a turning electrode 11 for connecting the two heat generating portions to each other; and a dielectric layer 21. The cantilever 4 is formed in a manner that the heat generation layer 20 is first formed, the conductor layer then is laminated on the heat generation layer 20; and lastly the dielectric layer 21 is laminated on the heat generation layer 20 and the conductor layer. A linear expansion coefficient of the dielectric layer 21 is set so as to be smaller than that of the generation heat layer 20. Moreover, the whole cantilever 4 is covered with a thin electrically insulating film (not shown) because of contact with ink. When the two heat generation portions of the cantilever 4 is energized to generate heat, the cantilever 4 is bent upward (toward nozzle 3) due to a difference between the linear expansion coefficients of the heat generation layer 20 and the dielectric layer 21 as shown in FIG. 8C. Thus, ink 7, with which the liquid chamber 2 is filled, is formed into a droplet 8 to be ejected from the nozzle 3.
A cantilever 4 is disclosed in Japanese Patent Laid-Open No. 2004-1517 in which the dielectric layer 21 is sandwiched between the two heat generation layers 20 and 20. First, the upper side heat generation layer 20 is energized so that the cantilever 4 is bent in a direction opposite from the nozzle 3. Next, the lower side heat generation layer 20 is energized so that the cantilever 4 is bent toward the nozzle 3 as shown in FIG. 8C. Thus, the droplets can be ejected by a large driving force.
Additionally, a trapezoid cantilever 4 is disclosed in Japanese Patent Laid-Open No. 2004-82733 in which the width of a fixed end 9 of the cantilever 4 is larger than that of a free end 10 thereof. The large driving force is also obtained by this constitution, and the droplets 8 can be properly ejected.
The ejecting portions of the print heads are arranged zigzag so as to be arranged at high density, as shown FIG. 9. The arrangement allows the nozzles to be arranged at short pitches even if the width of the liquid chamber 2 is increased for ink supply.
Problems of the thermomechanical actuator including the heat generation layers and dielectric layer constituting the cantilever will be described with reference to FIGS. 10A and 10B.
As shown in FIG. 10A, when the cantilever is bent to the maximum, that is, when the free end 10 of the cantilever 4 is brought closest to an inner wall (a roof portion) of the liquid chamber 2, the cantilever 4 is brought into an inclined state from the free end 10 to the fixed end 9. In this state, as indicated by an arrow F in FIG. 10A, a part of the pressure for ejecting the droplets escapes to the fixed end 9, and thus ejection energy cannot be entirely used, and energy efficiency sometimes becomes insufficient.
Additionally, since the cantilever 4 is inclined, the free end 10 of the cantilever 4 does not become parallel with a face 3a (a nozzle face 3a) on which an ejection opening of a nozzle 3 is formed. Accordingly, since an ejection pressure is not applied perpendicularly to the nozzle face 3a, an ejecting direction of the droplet 8 is inclined at the angle θ in relation to a nozzle face vertical line v as shown in FIG. 10A. Thus, a landing point of the droplet 8 ejected from the nozzle 3 may deviate from a target point. When the ejecting portions of the print head are arranged zigzag as shown in FIG. 9, the droplet 8 ejected from the adjacent nozzle 3 of the print head is inclined at the angle −θ in relation to a nozzle face vertical line v as shown in FIG. 10B. That is, if it is assumed that an odd number is assigned to the nozzle shown in FIG. 10A, and that an even number is assigned to the nozzle shown in FIG. 10B, the landing point of the droplet ejected from the nozzle 3 of the odd number may largely deviate from that of the even number.
The present invention was made to solve the above problems. It is an object of the present invention to provide a print head using the thermomechanical actuator, wherein the deviation of the landing point of the droplet ejected from the ejecting portion of the print head is removed even if the ejecting portions of the print head or the nozzles are arranged at a high density. Further, it is an object of the present invention to provide a print head using a thermomechanical actuator which has a high ejection efficiency.